1. Field of the Invention
This invention relates to grain-oriented electromagnetic steel sheets typically used as iron cores in electric generators and transformers, for example. More particularly, the invention relates to a grain-oriented electromagnetic steel sheet having a low ratio of iron loss in a weaker magnetic field to iron loss in a stronger magnetic field. Such sheets are suitably applicable to iron cores for small size electric generators and as E.I. cores for small scale transformers. The invention further relates to a process for production of such steel sheets.
2. Description of the Related Art
Grain-oriented electromagnetic steel sheets are used as iron core materials particularly for large-scale transformers and other electrical equipment. In general, such a steel sheet is required to have a low iron loss taken as the loss occurring upon magnetization of the steel sheet to 1.7 T at 50 Hz, and defined as W.sub.17/50. (W/kg). As a consequence, intensive research has been conducted with a view to reducing the value of W.sub.17/50. To prevent hysteresis loss among other iron losses, a certain technique is disclosed which causes the crystal grains of the finished steel sheet to be converged to the full extent possible to a {110} &lt;001&gt; orientation in which easy-to-magnetize axes &lt;001&gt; are arranged in a regular order in the rolling direction.
The grain-oriented electromagnetic steel sheet has been produced generally by use of complex process steps:
1) A slab 100 to 300 mm in thickness is subjected to heating and subsequently to hot rolling consisting of rough rolling and finish rolling, to prepare a hot-rolled sheet.
2) The hot-rolled sheet is cold-rolled once or twice or more times with intermediate annealing to reach a final sheet thickness.
3) The cold-rolled sheet is decarburization-annealed.
4) With an annealing separator coated over the decarburization-annealed sheet, finish annealing is performed to attain secondary recrystallization and purification.
5) Flattening annealing and insulating coating are applied to the finishing-annealed sheet, whereby a steel sheet product is obtained.
In the above method, those crystal grains directed to a {110}&lt;001&gt; orientation are allowed to grow through secondary recrystallization while in finishing annealing. To permit crystal grains to be grown in a {110}&lt;001&gt; orientation in an effectively conducted manner by means of secondary recrystallization, it is of importance that precipitation (commonly using an inhibitor) be made dispersible into a uniform and proper size, causing the inhibitor to prevent growth of crystal grains primarily recrystallized. One suitable inhibitor is typified by sulfides such as MnS, Se compounds such as MnSe, nitrides such as AlN and VN and so on, but they have a markedly weak tendency to dissolve into the steel.
In a conventional method of properly controlling such an inhibitor, the inhibitor has been completely solid-solubilized upon heating of the slab prior to hot rolling, followed by precipitation of such inhibitor in a subsequent hot rolling stage. In this instance, the slab needs to be heated at a temperature of about 1,400.degree. C. to produce a fully solid-solubilized inhibitor. This temperature is higher by about 200.degree. C. than that usually used in heating a steel slab. Slab heating at such a high temperature suffers from the following defects.
1) Substantial energy is consumed due to heating at an elevated temperature.
2) Melt scale and slab sagging tend to take place.
3) Excessive decarburization is likely to occur on the slab surface.
To solve the above defects 1) and 2) above, use of an induction heating furnace has been proposed for exclusive use in producing the grain-oriented electromagnetic steel sheet. However, such furnace causes a rise in energy cost. There is a keen demand for saving energy. To date, therefore, many persons skilled in the art have endeavored to practice slab heating at lower temperatures.
For instance, Japanese Examined Patent Publication No. 54-24685 discloses that the slab heating temperature can be set in a range of 1,050 to 1,350.degree. C. by incorporating into the steel such elements as As, Bi, Sb and the like, that segregate at a grain boundary, and by taking advantage of these elements as inhibitors. Japanese Unexamined Patent Publication No. 57-158332 teaches that the slab heating temperature can be lowered and the Mn content reduced with an Mn/S ratio of below 2.5, and that secondary recrystallization can be stably effected by addition of Cu. Additionally, Japanese Unexamined Patent Publication No. 57-89433 discloses conducting slab heating at a reduced temperature of 1,100 to 1,250.degree. C. by adding elements such as S, Se, Sb, Bi, Pb, B and the like together with Mn, and by taking a columnar structure ratio of the slab in combination with reduction of secondary cold rolling. However, since such known techniques are designed to omit AlN as an inhibitor having an extremely weak ability to dissolve into the steel, they fail to produce sufficient benefit from the inhibitors used, and hence create magnetic characteristics that are still far from acceptable. Eventually they have been used only for laboratory purposes.
In Japanese Unexamined Patent Publication No. 59-190324, a technique is taught in which pulse annealing can be employed at the time of annealing for primary recrystallization. This mode of production is also useful on a laboratory scale, but not on a commercial basis. Japanese Unexamined Patent Publication No. 59-56522 discloses heating a slab at a lower temperature with the Mn controlled to a content of 0.08 to 0.45% and with S less than 0.007%; Japanese Unexamined Patent Publication No. 59-190325 teaches stabilizing secondary recrystallization by further incorporation of Cr in the composition of 59-190325 cited above. While such prior art techniques are characterized with a small content of S, MnS is caused to solid-solubilize during slab heating, and such techniques have the disadvantage that upon use of their respective steel sheets for heavy weight coils, the resultant magnetic characteristics become irregular in the widthwise or lengthwise direction.
Japanese Unexamined Patent Publication No. 57-207114 discloses using a composition having a noticeably low content of carbon (C: 0.002 to 0.010%) in combination with a low slab heating temperature. This is attributable to the fact that where the slab heating temperature is low, absence of exposure to the austenite phase at stages from solidification to hot rolling is rather desirable for effecting subsequent secondary recrystallization. Such a low carbon content can avoid breakage during cold rolling, but nitridation is necessary in decarburization annealing in order to ensure stable secondary recrystallization.
With that technique in view, considerable technological development has been conducted on the basis that intermediate nitridation is employed. Namely, Japanese Unexamined Patent Publication No. 62-70521 discloses specifying finishing-annealing conditions and thus conducting slab heating at a low temperature by means of intermediate nitridation while in finishing annealing. Moreover, Japanese Unexamined Patent Publication No. 62-40315 teaches incorporating Al and N in amounts that cannot undergo solid solubilization during slab heating, thereby controlling the associated inhibitor in a proper state with reliance upon intermediate nitridation. Intermediate nitridation at the time of decarburization annealing, however, poses the drawback that it needs added equipment and hence increased cost. Another but serious drawback is that it is difficult to control nitridation in the step of finishing annealing.
On the other hand, one difficulty has of late arisen that the iron loss properties of a starting material do not always conform to those of the end-use product resulting from such material. It has been found, in fact, that in the case of iron cores for large-scale transformers, a starting material having a low value of W.sub.17/50 leads to an end-use product having excellent iron loss properties. Despite this finding, in the case of iron cores for electric generators of a small scale, or EI cores for use as small-scale transformers, the corresponding steel sheet has a complex magnetic flux running therein, with the consequence that the W.sub.17/50 value of the steel sheet does not necessarily match the iron loss properties of the resulting end product. As a result of the present energy crisis, reduction of energy waste must be reduced, and serious efforts have been made to decrease the iron losses of the end-use products. Any values of W.sub.17/50 as related to starting materials are not sufficient to correctly evaluate the end-use products. This has often created difficulty in selecting optimum materials to be used as starting materials.
In reducing the iron loss of a starting material, it is generally known to provide a method in which electrical resistance is increased by addition of Si that acts to effectively decrease eddy-current loss, or a method in which a steel sheet is decreased in thickness, or a method in which crystals are decreased in grain sizes, or a method in which magnetic flux is improved in density by converging crystal orientations to {110} &lt;001&gt; to a great extent. The method of improving a magnetic flux density, amongst the above methods, has been widely studied to date. In Japanese Patent Publication No. 51-2290, for example, it is disclosed that with Al added as an inhibitor component to steel, slab heating is effected at a high temperature of above 1,300.degree. C., finishing rolling for hot rolling is conducted at a high temperature for a short period of time, and hot rolling is done at a final temperature of above 980.degree. C. Japanese Patent Publication No. 46-23820 discloses that with Al added to steel, particulate AlN is allowed to precipitate by annealing the hot-rolled steel sheet at a high temperature of 1,000 to 1,200.degree. C. and by subsequently quenching the annealed steel sheet; also the quenched steel sheet is subjected to cold rolling with a high reduction of 80 to 95%. Thus, a steel material is made available which offers a noticeably high magnetic flux density of 1.95 T at B.sub.10, and a low iron loss.
In regard to the method which is designed to achieve improved magnetic flux density by arrangement of crystal orientations and which has conventionally been used in reducing W.sub.17/50, it cannot be said that such method is effective for improving the iron loss properties of EI cores or iron cores for small-scale generators. One reason for this is that the magnetic flux distributed in a steel sheet is complex, as in the case of the EI cores.
To decrease the iron loss without use of the magnetic flux density-improving method, there have been considered a method in which Si is added in a large amount, a method in which the steel sheet is decreased in thickness, and a method in which crystals are reduced in grain size. In the Si content-increasing method, excess Si leads to impaired rolling of and diminished workability of a steel sheet. The steel sheet thickness-decreasing method produces a sharp rise in production cost.